1. Field of the Invention
This invention relates to photoelectrochemical (PEC) etching for laser facets.
2. Description of the Related Art
In laser diodes, one of the most difficult processing steps is the formation of high quality laser facets. The reflectivity of these facets needs to be high, the angle of the facet must be precisely determined, and ion damage should be kept to a minimum to ensure the quality of the active region. Typically, laser facets are formed by wafer cleaving, which leads to high quality facets but relies on the presence of a convenient cleave plane in the desired direction. In situations where cleaved facets are not possible (e.g., the lack of an appropriate cleavage plane, the existence of a substrate that is lattice-mismatched from the active area material, the desire to form integrated laser arrays), dry etching may be used. However, it is challenging to both produce mirror-smooth surfaces, and achieve the correct facet angle with dry etching. In addition, ion-assisted dry etching can damage the active material, degrading the laser performance. The use of simple (i.e. non PEC) wet etching to form laser facets also has limitations, since many wet etchants are crystallographic, producing etched sidewalls at angles that are determined by crystallographic planes which are not necessarily the optimal angle for a laser facet. This is illustrated in FIGS. 1 and 2.
FIGS. 1A and 1B describe isotropic etching of Silicon (Si), wherein FIG. 1A is a micrograph of an isotropic etched profile and FIG. 1B is a schematic of the isotropic etched profile, with both figures showing the etched Si 100 and SiO2 etched mask 102. (These figures can be found in the publication by Madou, entitled “Fundamentals of Microfabrication,” CRC Press, 2nd Edition, p. 210.) These figures illustrate that “simple” wet-etched profiles may not be suitable for laser facets. Specifically, these figures illustrate that, even using non-PEC wet etches, without ion damage, there is no guarantee that the etched angle or profile required for a laser facet can be achieved.
FIGS. 2A and 2B describe crystallographic etching of Si, wherein FIG. 2A is a cross-section schematic and FIG. 2B is a top view schematic of an anisotropic etched profile, both showing etched Si 200 and a SiO2 etched mask 202. (These figures can be found in the publication by Campbell, entitled “The Science and Engineering of Microelectronic Fabrication,” Oxford University Press, 2nd Edition, p. 528.) These figures also illustrate a cavity 204 and pyramidal hole 206 formed in the (100) Si wafer 200 with an anisotropic etchant. As noted in the figures, the surface of the SiO2 etched mask 202 is oriented along the <100> plane, a sidewall of the cavity 204 is oriented along the <111> plane, and a sidewall of the pyramidal hole 206 is etched at an angle φ=54.74° relative to the surface of the SiO2 etched mask 202.
For many common laser geometries, including c-plane gallium nitride (GaN) grown on sapphire, it is difficult to form cleaved laser facets (one reason for this is the sapphire substrate is lattice mismatched to the GaN). In fact, in all cases where the GaN is grown on a lattice mis-matched, misoriented substrate, the issue of cleaved facets becomes challenging.
Recently, semipolar lasers grown along the (11-22) direction have attracted interest because it is possible to get high indium content quantum wells on this crystal face, extending the range of wavelengths possible with GaN-based lasers, and in particular, enabling the development of lasers that emit longer-wavelength green light. Unfortunately, there is no convenient cleavage plane for these devices, so etched facets must be used instead.
A method for wet etching of laser facets, where the geometry of the etched sidewall can be controlled by factors other than the crystal structure of the material itself, would be desirable for all crystal planes because it would give improved control over device size and shape. However, such a method would be especially desirable for semipolar orientations where wet or dry etching is the only option. The present invention satisfies this need.